Power Supply Connection - Texas Instruments 2000 Series Reference Manual

February 2002

3.2 Power Supply Connection

Ground pins for the logic/receiver part and the transmitter power stage are not
directly connected internally, the two different grounds having to be connected to
each other externally.
The only internal connection is via resistor R_GND, in order to avoid floating
grounds if these grounds are accidentally not connected to each other externally.
This is necessary for two reasons:
1. A high resistivity of the GNDP pins could cause a voltage drop across these
pins, due to high transmitter power stage current (this does not apply to the
supply pins of the logic section). If the grounds were connected to each other
internally, this would also lift the internal logic ground and cause logic level
compatibility problems with the Control Module (see Figure 9).
2. In order to provide greater flexibility when using long supply lines.
Long VSP supply lines between the RA-RFM and the Control Module cause a
voltage drop across this supply line (again due to high transmitter power stage
supply current). This voltage drop would also lift the logic ground and cause
logic level compatibility problems with the Control Module. This can be avoided
by connecting the grounds externally in any of three different ways (see also
Figure 9) as described below:
•
For cable lengths of up to 0.5 m between RA-RFM and Control Module, the
RFM ground pins GND and GNDP must be connected at the Control Module,
as shown in Figure 9. The grounds for the VSP, VSL and the Control Module
supply are connected together at a common ground. Alternatively, if the
voltage drop across the VSP supply line is less than 0.5 V (likely in this case),
the ground pins GND and GNDP may be connected together at the RA-RFM. If
the system has a TI-RFID Control Module, the RA-RFM ground pins GND and
GNDP are already connected together correctly on the Control Module. When
using a customer-specific controller, care must be taken to connect the RA-
RFM ground pins GND and GNDP to an appropriate ground on the controller.
•
For cable lengths of between 0.5 m and 2 m, the RFM ground pins GND and
GNDP must be connected together at the Control Module in order to avoid
logic level compatibility problems caused by the voltage drop across the VSP
supply lines. Connecting the ground pins at the RA-RFM is not permitted since
this would lift the logic ground level.
•
Cable lengths longer than 2 m are not recommended.
demands cabling longer than 2 m, the logic signal connections between the
RA-RFM and the Control Module should be done via a differential interface (for
example RS422). Due to different ground potentials at different locations it may
also be necessary to provide galvanic separation of the interface signals by, for
example, opto-couplers. In this case, to avoid problems with difference
voltages between GND and GNDP, these pins must always be connected
directly at the RA-RFM. As shown in Figure 9 a shorting bridge is necessary for
this purpose, situated as close as possible to the RA-RFM.
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Installation
If the application